Function
of piles
Piles
are columnar elements in a foundation which have the function of transferring
load from the superstructure through weakly
compressible strata or through water, onto stiffer or more compact and less
compressible soils or onto the rock. They
may be required to carry uplift loads when used to support tall structures
subjected to overturning forces from winds or waves. Piles used in marine
structures are subjected to lateral loads from the impact of berthing ships and
from waves. Combinations of vertical and horizontal loads are carried where
piles are used to support retaining walls, bridge piers and abutments, and
machinery foundations.
Historical
The driving of
bearing piles to support structures is one of the earliest examples of the art
and science of a civil engineer. In Britain, there are numerous examples of
timber piling in bridge works and riverside settlements constructed by the
Romans. In medieval times, piles of oak
and alder wood were used in the
foundations of the great monasteries constructed in the fenlands of East
Anglia. In China, timber piling was used by the bridge builders of the Han
Dynasty (200 BC to AD 200). The carrying capacity of timber piles is limited by
the girth of the natural timbers and the ability of the material to withstand
driving by hammer without suffering damage due to splitting or splintering.
Thus primitive rules must have been established in the earliest days of piling
by which the allowable load on a pile was determined from its resistance to
driving by a hammer of known weight and with a known height of the drop. Knowledge was also accumulated regarding
the durability of piles of different species of wood, and measures taken to
prevent decay by charring the timber or by building masonry rafts on pile heads
cut off below water level.
Timber,
because of its strength combined with lightness, durability, and ease of cutting and handling remained the only material used for piling until
comparatively recent times. It was replaced by concrete and steel only because
these newer materials could be fabricated into units that were capable of
sustaining compressive, bending and tensile forces far beyond the capacity of a
timber pile of like dimensions. Concrete, in particular, was adaptable to
in-situ forms of construction which facilitated the installation of piled
foundations in drilled holes in situations where noise, vibration, and ground have
had to be avoided.
Reinforced
concrete, which was developed as a structural medium in the late nineteenth and
early twentieth centuries, largely replaced timber for high-capacity piling for
works on land. It could be precast in various structural forms to suit the
imposed loading and ground conditions, and its durability
was satisfactory for most soil and immersion conditions. The partial
replacement of driven precast concrete piles by numerous forms of cast in-situ
piles has been due more to the development of highly efficient machines for
drilling pile bore-holes of large diameter and great depth in a wide range of
soil and rock conditions, than to any deficiency in the performance of the
precast concrete element.
Steel has been used to an increasing extent for piling due
to its ease of fabrication and handling and its ability to withstand hard
driving. Problems of corrosion in marine structures have been overcome by the
introduction of durable coatings and cathodic protection.
No comments:
Post a Comment